1. Field of the Invention
The present invention relates generally to a fluid-filled cylindrical vibration damping device adapted to provide a vibration damping or isolating effect based on flows or resonance of a fluid contained therein. More particularly, this invention is concerned with such a fluid-filled cylindrical vibration damping device which is capable of effectively damping or isolating the input vibrations over a wide frequency range, based on the flows of the fluid.
2. Discussion of the Related Art
As one type of a vibration damping device interposed between two members of a vibration system, there is known a fluid-filled cylindrical vibration damping device which includes (a) an inner sleeve member, (b) an outer sleeve member disposed radially outwardly of the inner sleeve with a suitable radial distance therebetween, (c) an elastic body interposed between the inner sleeve member and the outer sleeve member for connecting these two sleeve members, (d) a pressure-receiving chamber which is partially defined by the elastic body and filled with a non-compressible fluid such as water, (e) an equilibrium chamber which is partially defined by a flexible diaphragm and filled with the non-compressible fluid, and (f) an orifice passage for permitting flows of the fluid between the pressure-receiving and equilibrium chambers. Such a vibration damping device is used as an engine mount for a motor vehicle. The cylindrical vibration damping device of this type is capable of exhibiting an excellent vibration damping or isolating effect based on the flows or resonance of the fluid which is forced to flow through the orifice passage upon application of the vibrations between the inner and outer sleeves.
Generally, the engine mount for the motor vehicle is required to exhibit different damping or isolating characteristics with respect to the input vibrations over a wide frequency range since the frequency of the vibration to be damped changes depending upon the running condition of the vehicle, for instance. For example, the engine idling vibrations applied to the engine mount while the vehicle is stationary with the engine placed in its idling state have a plurality of frequency order components. In addition, in a motor vehicle equipped with an automatic transmission, the frequency of the input vibration changes with a change of the engine speed in different positions of the transmission. In general, the engine mount is required to exhibit a vibration isolating effect with respect to the input vibrations over a wide frequency range of 20-40 Hz.
However, the above-described fluid-filled cylindrical vibration damping device exhibits an effective vibration damping or isolating effect based on the fluid flows through the orifice passage, with respect to only the input vibrations in a relatively narrow frequency range which is determined depending upon the length and cross sectional area of the orifice passage. Further, the dynamic spring constant exhibited based on the resonance of the fluid flowing through the orifice passage is relatively low when the vibration frequency is lower than the resonance frequency of the fluid flowing through the orifice passage, but is abruptly increased in the neighborhood of the resonance frequency. Namely, the dynamic spring constant of the engine mount considerably increases when the input vibration has a frequency higher than the frequency to which the orifice passage is tuned, whereby the vibration damping effect to be exhibited by the engine mount is considerably deteriorated.
Thus, the conventional fluid-filled cylindrical vibration damping device is not capable of providing a vibration isolating effect exhibited by the low dynamic spring constant based on the fluid flows, over a wide frequency range to be required. Accordingly, the conventional vibration damping device does not provide the desired vibration damping or isolating characteristics.